1. Field of the Invention
The invention is directed to a clutch device for connecting a motor vehicle drive with a multiple-speed transmission via at least two friction clutches which are connected with associated shift transmission input shafts by respective driven sides such that the clutches can be engaged or disengaged independently of one another.
2. Description of the Related Art
A clutch device of this kind is known, for example, from EP 0 931 951 A1. The clutch device serves to connect the drive of a motor vehicle with a multiple-speed transmission via two friction clutches which are preferably automatically actuated, a clutch release system being associated with each of these friction clutches so that the two friction clutches can be engaged or disengaged independently from one another. One of the friction clutches is arranged on a central transmission input shaft so as to be fixed with respect to rotation relative to it, while the other friction clutch engages at a second transmission input shaft so as to be fixed with respect to rotation relative to it, this second transmission input shaft, constructed as a hollow shaft, enclosing the central transmission input shaft. Clutch devices of this type, which are known in technical circles as xe2x80x9cdouble clutchesxe2x80x9d, particularly when combined with a multiple-speed shift transmission, offer the possibility of carrying out shifting processes between two respective gear ratio speeds of the transmission without interruption of tractive forces.
The laid open application cited above addresses not only the advantages of such a clutch device, but also indicates the actual problem, albeit only indirectly: It is stated, for example, that both clutch disks can be acted upon jointly during particularly difficult starting processes. Such starting processes occur, for example, when the accelerator pedal is deflected to the region of its stop, while the motor vehicle is kept essentially stationary at the same time by applying the maximum braking force until the clutch has reached its optimal transmission point which, when exceeded, would cause the drive to stall. If the braking action is canceled at this time, the vehicle is started with maximum acceleration. While starting processes of this kind are common primarily in car racing, they may be required particularly in motor vehicles with a relatively weak engine under extreme starting conditions, for example, when starting on an obstruction. This results in high slippage which brings about extensive development of heat. Consequently, not only is there the problem that this heat must be carried away from the area of extension of the friction clutch, but increased wear of the friction clutches must also be taken into account. Further, heating of this kind leads to changes in the coefficient of friction at the friction clutches so that the control of the two release mechanisms, and therefore of the two friction clutches relative to one another, can be appreciably impaired. This is incompatible with a problem-free torque transmission in shifting processes in the shift transmission without interruption of tractive force and without jerking during shifting. Specifically, as a result of inaccuracies in functional matching of the two friction clutches relative to one another caused by heat, a torque ratio not intended in the shifting process is applied to the two transmission input shafts, which can lead to shifting processes in the shift transmission under load. This can overtax the synchronization in the shift transmission so that, aside from the disadvantages with respect to efficiency, the shift transmission can be damaged to the point of complete failure.
Also problematic in clutch devices of this kind are starting processes which are either carried out in opposition to an inclination, wherein the motor vehicle must be prevented from rolling backward, or which are used when parking at the lowest possible speed for precise positioning of the motor vehicle in a parking space. The first operating state mentioned above is referred to in technical circles as xe2x80x9chill-holdingxe2x80x9d, the latter operating state is referred to as xe2x80x9ccreepingxe2x80x9d. Both starting processes have in common that the friction clutch is operated, sometimes without actuation of the accelerator, over longer period of time with slippage. Although the torques to be transmitted in such starting processes lie well below those in the operating condition described above, an intensive heating of the friction clutches can occur, resulting in the problems detailed above.
Gear-shifting strategies and shifting processes for double-clutch transmissions based on the aimed for adjustment of clutch slip have been suggested (German reference DE 196 31 983 C1) with consequent generation of friction heat. Depending on driving behavior, overheating problems of the type mentioned above cannot be ruled out.
The risk of intensive overheating exists not only in a dry friction clutch, but can also occur in so-called xe2x80x9cwetxe2x80x9d friction clutches, preferably in the form of a disk or plate clutch which is arranged within a viscous medium such as hydraulic fluid. In this connection, reference is had to German reference DE 198 00 490 A1, for example, in which two plate clutches are arranged in viscous medium, one being provided for forward driving and the other for driving in reverse. The object of German reference DE 198 00 490 A1 is adequate cooling of wet friction clutches of this type, the viscous medium being utilized for this purpose. However, in spite of the liquid cooling, heating of the friction clutches constitutes a considerable problem because, on the one hand, the viscous medium overheats when flowing through flow passages in the form of channels usually provided in friction surfaces of the friction clutch and can accordingly be destroyed and, on the other hand, due to the building up of an opposing pressure between the friction surfaces of two adjacent plates with intensive flow through these grooves, capacity of the friction clutch to transmit torque is reduced and the problem of overheating is therefore exacerbated due to increased slippage. Particularly in plate clutches, this problem can result in that the friction surfaces can no longer separate from one another completely after a disengaging process and, consequently, considerable drag torques can sometimes reach the shift transmission because one of the friction clutches is already closed, while the other has not yet completely opened.
It is the object of the invention to construct a clutch device in such a way that an intensive heating is prevented in the area of the friction clutches even under unfavorable operating conditions such as in problematic starting processes in a motor vehicle.
This object is met according to the invention by producing an operative connection between one of the friction clutches and a hydrodynamic clutch, the latter can be activated as a substitute for the friction clutch that is in a working connection with the hydraulic clutch during difficult starting processes such as, for example, when overcoming obstacles, when starting on hills using the xe2x80x9chill-holderxe2x80x9d function and when xe2x80x9ccreepingxe2x80x9d for parking the motor vehicle in a narrow parking space. In this respect, the hydrodynamic clutch is advantageous in a two-fold sense: first, while starting processes with a hydrodynamic clutch, even under full load, lead to a temporally limited relative movement of the driving memberxe2x80x94usually formed by an impeller wheelxe2x80x94with respect to the driven member having a turbine wheel and therefore result in losses for reasons pertaining to flow, the heat development occurring in this connection remains within strict boundaries, especially since the transmission of torques in hydrodynamic clutches of this type entails a comparatively high volume flow of viscous medium circulating between the impeller wheel and turbine wheel. As soon as the turbine wheel has almost reached the rotational speed of the impeller wheel, the hydrodynamic clutch operates with comparatively low losses. Consequently, by connecting the driven member of the hydrodynamic clutch, that is, the turbine wheel, with the driven side of one of the friction clutches and with one of the transmission input shafts of the shift transmission, the torque provided by a drive unit can be transmitted directly to this transmission input shaft by circumventing the friction device connected with the hydrodynamic clutch. The transmission input shaft takes part in the above-described acceleration of the turbine wheel by means of the impeller wheel by way of the volume flow of viscous medium running therebetween. The corresponding friction clutch is either disengaged during the activation of the hydrodynamic clutch or is operated in combination with the hydrodynamic clutch in order to distribute the output loss to two clutches with a distribution ratio which can be predetermined.
In case the friction clutches are arranged with a radial offset relative to one another to economize on axial installation space, it is advantageous to connect the radially outermost friction clutch with the hydrodynamic clutch. The reason for this is as follows: The hydrodynamic clutch is preferably connected with the transmission input shaft to which the highest torques can be applied, that is, with that transmission input shaft to which the first gear speed and the reverse gear are allocated. Because of this, in a shift transmission with two transmission input shafts, the distribution is preferably effected such that gear speeds 1, 3 and 5 and the reverse gear are allocated to the first transmission shaft cooperating with the hydrodynamic clutch and gear speeds 2, 4 and 6xe2x80x94in a six-speed shift transmissionxe2x80x94are allocated to the second transmission input shaft.
In contrast to extreme starting processes such as starting under full load or with the xe2x80x9chill-holderxe2x80x9d or xe2x80x9ccreepxe2x80x9d function, simple starting processes can be carried out by circumventing the hydrodynamic clutch by means of one of the friction clutches. Since the radial outer friction clutch in comparison to the friction clutch arranged farther radially insidexe2x80x94assuming an identical number of plates in both friction clutchesxe2x80x94is better suited for transmitting higher torques than friction clutches located farther inward radially, the radial outermost friction clutch is preferably connected with the transmission input shaft at which the hydrodynamic clutch also engages. Accordingly, a simple starting process can be carried out with this friction clutch, so that the efficiency of the clutch device and therefore of the entire drive train which, aside from the clutch device, also comprises the drive and the shift transmission, is increased compared with a construction of the clutch device in which every starting process is carried out via the hydrodynamic clutch.
An alternative torque transmission via the hydrodynamic clutch or via one of the friction clutches requires that all clutches can be switched on or off regardless of whether or not they are hydrodynamic clutches or friction clutches. In the latter case, for this purpose, a working connection is established or canceled between friction surfaces of adjacent clutch members such as the outer plates and inner plates of a multiple-plate clutch. In contrast, the hydrodynamic clutch can be shifted as described in the following:
To deactivate the hydrodynamic clutch, it is possible to throttle down or even halt a volume flow of viscous medium which, when the hydrodynamic clutch is activated, is preferably guided via a flow inlet provided at its radial inner side into the hydrodynamic circuit, while the possibility is provided at the same time on the radial outer side for viscous medium contained in the hydrodynamic circuit to flow off quickly. For this reason, the hydrodynamic clutch is covered by an enclosure which substantially covers the hydrodynamic circuit and preferably has an opening on the radial outer side through which viscous medium can flow out of the hydrodynamic circuit due to centrifugal force so that this hydrodynamic circuit can be emptied. This opening is preferably provided with a through-flow control in which, for example, the through-flow cross section of the opening is increasingly reduced so that the emptying of the hydrodynamic circuit is slowed down. A through-flow control of this type is basically a flow limiter. It acts in a particularly flexible manner when it is adjustable as continuously as possible via an actuator, e.g., an electromagnet. An alternative possibility consists in through-flow controls which act by means of geometric shaping, for example, by forming the edge defining the opening in the enclosure as a nozzle or a diaphragm. To summarize, the emptying rate and therefore the period of time for deactivating the hydrodynamic clutch can accordingly be determined by means of this through-flow control, while the volume flow at the flow inlet is at least reduced simultaneously. Conversely, an activation of the hydrodynamic clutch by introducing or amplifying a volume flow of viscous medium via the flow inlet can be achieved, preferably with at least partial reduction of flow in the area of the radial outer opening, since the hydrodynamic circuit can accordingly be filled with viscous medium within a time period which can be predetermined.
The enclosure of the hydrodynamic clutch which was mentioned above and which is necessary for operation is preferably used to connect with a crankshaft of the drive, for example, via a toothing, so as to be fixed with respect to relative rotation. The enclosure is therefore guided out of the housing enclosing the clutch device on the drive side, while the rest of the clutch device remains inside the housing. The housing is protected at least substantially against escape of viscous medium by means of a cover and a seal which is provided between the cover and the enclosure which is guided outward on the drive side.
According to another further development, the hydrodynamic clutch is connected via the enclosure with a torsional vibration damper which acts, in turn, on the drive side of the associated friction clutch.
The hydrodynamic clutch can be constructed as a hydraulic clutch with exclusive use of impeller wheel and turbine wheel, but can also be constructed as a hydrodynamic torque converter when a stator wheel is additionally used between the two wheels mentioned above. In the latter case, it is necessary to provide the possibility for supporting the stator wheel relative to the forces brought about by flow between the impeller wheel and turbine wheel; this support is carried out by means of a supporting shaft in the transmission housing of the shift transmission, which supporting shaft is central relative to the axis of rotation of the clutch device.
According to a further development, all friction clutches are connected with a hydraulic pump either by their drive side or by their driven side, so that the friction clutches are given the added function of a pump drive. By means of this hydraulic pump, pressure channels can be generated for building up a control pressure for the respective friction clutch on one hand and flow channels can be generated for building up a volume flow to the friction clutches as well as to the hydrodynamic clutch on the other hand. Pressure chambers in the housing of the clutch device can be acted upon via the pressure channels, specifically, in such a way that a piston of the friction clutch associated with the pressure chamber is deflected against the action of an axial spring, and the friction clutch which is preferably constructed as a multiple-plate clutch is accordingly pressed against a shoulder in that the outer and inner plates are pulled together and the friction clutch is accordingly engaged. When overpressure is built up in one of the pressure channels and accordingly in one of the pressure chambers, a reduction in pressure must be generated simultaneously in the other respective pressure channel and a pressure drop must be generated in the associated pressure chamber, so that another piston associated with the other friction clutch is pulled back into its initial position by the action of another axial spring and the associated friction clutch is therefore disengaged. For the next shifting processes, however, the pressure buildup and pressure drop in the individual pressure channels are reversed. At the same time, the hydraulic pump generates a volume flow which preferably flows through a flow channel radially between the two transmission input shafts, so that at least one of the transmission input shafts, that is, the radial outer transmission input shaft, must be constructed as a hollow shaft which encloses the inner transmission input shaft with radial play to form a flow channel with an annular cross section. A volume flow of viscous medium reaches the housing of the clutch device through this flow channel and arrives, in particular, in the area of extension of the friction clutches, wherein the latter preferably have flow passages in the friction facings associated with the individual plates for the passage of the volume flow. Flow passages of this type may have any shape. A great many different constructions for flow passages of this type are known from the patent literature. According to German reference DE 44 32 624 C1, such flow passages can have a winding shape, while U.S. Pat. No. 5,094,331 shows preferred radial flow passages and U.S. Pat. No. 5,101,953 discloses flow passages between which protuberances are formed in a waffle pattern. The construction of these flow passages is designed in accordance with the purpose for which the friction clutches are to be used.
One of the flow channels leads past the area of extension of the friction clutch to the hydrodynamic clutch and serves to supply the latterxe2x80x94at least occasionallyxe2x80x94with a volume flow of viscous medium which serves to activate the hydrodynamic clutch in the manner already detailed above, especially during starting processes. The transmission input shaft enclosing the axis of rotation of the clutch device is constructed as a hollow shaft, so that this flow channel extends centrally. This design is advantageous in a preferred manner when the hydrodynamic clutch is constructed as a hydraulic clutch. However, when the hydrodynamic clutch is constructed as a hydrodynamic torque converter, the flow channel is preferably provided with an annular cross section radially between the hollow, radially innermost transmission input shaft and supporting shaft because of the need to support the stator wheel in the transmission housing of the shift transmission and due to the fact that a supporting shaft extending between a freewheel of the stator wheel and the transmission housing is consequently guided through, preferably centrally.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of the disclosure. For a better understanding of the invention, its operating advantages, and specific objects attained by its use, reference should be had to the drawing and descriptive matter in which there are illustrated and described preferred embodiments of the invention.